Run a piston engine on a live pressure-volume diagram, trace the four strokes into a closed loop, and bank the enclosed area as useful work.
A heat engine cycles through four strokes, heat, expand, cool, compress, drawing a closed loop on a P–V diagram. The shaded area inside the loop is your work output. A ceiling rail marks the maximum work the second law of thermodynamics allows.
Carnot makes the thermodynamics of an ideal heat engine playable: every loop you draw obeys the real relations.
Four strokes drawing a closed loop on a P–V diagram, with the enclosed area as work output, is the Carnot cycle — the idealised engine against which every real one is measured.
Reading work off the loop's area is the original device.
The area is the work, which is why the diagram is drawn at all. Carnot's result is that efficiency depends only on the two temperatures between which the engine runs, setting a ceiling no design can beat — one of the few genuinely absolute limits in engineering.
Carnot makes the thermodynamics of an ideal heat engine playable: every loop you draw obeys the real relationship between work, heat and efficiency. It's an original, free browser game with three difficulties and daily plus practice modes.